A. Field of the Invention
In at least one aspect, the present invention relates to film deposition useful in the installation of a worldwide fiber-optic network, which is in progress, capable of handling levels of data transmission inconceivable only several years ago. As a result of this network, the Internet is less than half a decade away from being a more useful tool than the computers which navigate it. Advanced thin film coatings have emerged as the enabling technology to control transmission and reflection of selected wavelengths of light. From this, and other technical achievements, existing optical fibers will accommodate the increase in bandwidth that is required over the next 3–5 years.
Dense Wavelength Division Multiplexer (DWDM) systems enable information to be delivered inside fiber-optic cables at multiple wavelengths. The increase in the bandwidth is limited only by the number of wavelengths which can be superimposed on the fiber. Current state-of-the-art DWDM can multiplex/demultiplex approximately 40 channels. Ultimately more than 1000 channels will be possible. During transmission, information is packaged within pulse modulated carriers at specific wavelengths and superimposed (multiplexing) on the fiber. During reception, the carriers must be separated (demultiplexing). Optical component technology such as DWDM is critical in order to achieve the bandwidth necessary for future interactive services, such as “video on demand,” and has prompted multi-billion dollar strategic acquisitions such as OCLI®, NetOptix™, and XROS™.
The most widely used technology for demultiplexing in DWDM systems is thin film-based. Multilayered, thin dielectric coatings are comprised of 150–200 layers with individual optical layer thickness equal to multiples of ¼ of the wavelength to be transmitted (known as dielectric interference filters.) A collection of such filters, coupled together, each differing slightly in design to allow light transmission of different wavelengths, and “connected” to a fiber-optic cable, enables the multiplexing (superposition) and demultiplexing (separation) of multiple wavelengths of laser light containing digital information.
Current thin film demultiplexer filters are produced with accepted yields of less than 5%, due to the complexity and uniformity requirements of the filter designs. Coating equipment used for complex optical coatings are not optimally tooled to provide necessary uniformity for this application, and are therefore unable to produce a high throughput of certified filters. A large-area, ion assisted, electron-beam evaporation system has been designed which utilizes a novel fixture assembly resulting in a substantial improvement in yields. The system employs several modifications to conventional deposition configurations and processes to enable high throughput of narrow band pass filters for multiplexers (muxes) and demulitplexers (demuxes) in DWDM systems.
B. Description of the Related Art
Thin film coatings designed to permit light transmission/reflection over narrow (0.1–25 mm) and broad (>25 mm) band passes are typically comprised of multiple layers of two or more optically matched “high” index and “low” index materials. The individual layer thickness and number of layers will ultimately define the optical performance of the filter. Typical “high performance” narrow band filters may have more than 100 individual layers.
High performance dielectric thin film optical filters are produced in volume for state-of-the-art muxes and demuxes used in DWDM systems. These filters are produced with materials such as SiO2 and Ta2O5 deposited with processes such as ion beam sputter deposition (ISBD) and ion-assisted deposition (IAD). Filters produced with these processes are stable under adverse environmental conditions.
Thickness uniformity is critical in any optical filter application. Optical coating systems are typically designed to produce coatings with thickness uniformity of approximately 0.1% variation over the substrate area. This level of thickness control is insufficient for multilayered coatings designed for DWDM. Layer thickness determines wavelength and amplitude (loss) of transmitted light, therefore, accurate thickness determination and reproducibility are crucial.
In practice, tens of substrates (approximately 6″ square) are coated with multilayer filters designed for DWDM in “traditional” IBSD or IAD systems. A typical IAD production coating system can be approximated by a 60″ cube with a fixture assembly located at the top of the vacuum chamber as shown schematically in FIG. 1A. The planetary fixture assembly 34 is designed for thickness uniformity described above and can accommodate approximately sixteen (16) to twenty-four (24) 6″ square or round substrates. As many as five QCMs (quartz crystal monitors) and an optical monitor are positioned about the chamber to monitor deposition rate and optical layer thickness. The quartz monitors are calibrated prior to production. Deposition rate incident on the substrate assembly is determined by sampling each monitor and averaging.
Two or three electron guns are employed to reduce the deposition time as shown in FIG. 1A. The filter is comprised of more than 150 alternating layers, and the source material must be preheated before each layer is deposited. The preheating process can take from 30–120 seconds (0.5–2.0 minutes) which would add up to (0.5–2.0)×150=75–300 additional minutes to the deposition cycle if a single gun was used.
The substrates are diced into thousands of ˜1 mm squares (called dies or chips). Every coated die is tested for performance to determine which ones, if any, meet requirements. Major manufacturers such as OCLI® have reported production yields of less than 5%. The demand for such filters is approaching 1 million per month. This demand will not be met with the current system configurations without a significant increase in capital equipment to increase capacity. Customers for the filters have relaxed requirements and settled for inadequate performance to continue with installation of DWDM systems
A patent pending high yield fixture, called the Vornado™, has been designed to produce demux filters for DWDM systems with greater than 25% accepted yield. The design is comprised of a disk (approximately 8.5″ in diameter) with a concentric multi crystal QCM and a dedicated “clam” shutter arrangement. The disk rotates at greater than 1000 rpm during operation to insure uniform deposition of material at typical coating deposition rates of 0.2–0.5 nm/s. The Vornado™ is capable of yielding approximately 1000 filters per deposition.